Warm White Light Emission Research Articles

Warm white light emission of apatite-type compound Ca4Y6O(SiO4)6 doped with Dy3+

A series of Ca4Y6O(SiO4)6: x%Dy3+ (x = 1, 3, 5, 7, 9) phosphors were synthesized successfully via a conventional solid-state method at high temperature. The X-ray diffraction results show that all obtained samples are the pure hexagonal crystal structure of the space group P63/m (No.176). The blue (4F9/2 → 6H15/2) and yellow (4F9/2 → 6H13/2) emissions of Ca4Y6O(SiO4)6:7% Dy3+ phosphors are of such a ratio to produce almost perfect white light with CIE coordinates of (0.330, 0.339). The correlated color temperature is about 5600 K, close to that of the midday sunlight (5500 K). In addition, both the emission intensity and the CIE coordinates exhibit an excellent thermal stability in the temperature range of RT ∼ 300 °C. Our study illustrates that Ca4Y6O(SiO4)6:7%Dy3+ may serve as potential warm white lighting phosphors in NUV-based solid state lighting and optical display applications.

Novel highly efficient single-component multi-peak emitting aluminosilicate phosphors co-activated with Ce3+, Tb3+ and Eu2+: luminescence properties, tunable color, and thermal properties.

A series of emission-tunable Ce3+/Tb3+/Eu2+ doped Ca2(Mg0.75Al0.25)(Si1.75Al0.25)O7 (denoted as CMAS) phosphors have been synthesized via a high temperature solid-state reaction method. The luminescence properties, color tuning, quantum yields (QYs), energy transfer of Ce3+ to Tb3+/Eu2+, thermal stability, performance of LED devices and ratiometric temperature sensing application have been systematically investigated, respectively. Importantly, through the study of thermal stability, we found that Ce3+ and Tb3+ co-doped samples were suitable for WLED applications, while Ce3+ and Eu2+ co-doped samples were suitable for temperature sensing applications. Due to the energy transfer, Ce3+/Tb3+ co-doped samples had high luminous efficiency and the quantum efficiency of more than 80% could be achieved. Their emission colors can modulate from blue to green. In addition, on the basis of the evaluation of the as-fabricated white LED lamps via selecting the corresponding phosphors, the CCT can reach 4275 K and the CRI can increase to 86.8, indicating that this series of phosphors can act as potential color-tunable phosphors for possible applications in ultraviolet light based white LEDs. Importantly, it is found that the fluorescence intensity ratio of CMAS : 5%Ce3+,0.5%Eu2+ displays linear correlation with temperature in a wide range of 253-373 K with a high sensitivity of 2.49% K-1, indicating that it could be a good candidate for ratiometric optical thermometry.

Novel single component tri-rare-earth emitting MOF for warm white light LEDs.

Single-phase white phosphors could overcome many drawbacks of traditional phosphors, and warm white light phosphors have been considered suitable for indoor illumination applications. Thus, synthesizing new single-phase warm white phosphors is significant for the production of LEDs. Based on the above considerations, a novel single component warm white light phosphor SmxTbyDy0.2-x-y-metal organic frameworks (MOFs) has been prepared successfully in this study. The compound Sm0.1Tb0.04Dy0.06-MOF shows ideal warm white light emission with CIE coordinates (0.333, 0.3522), a color rendering index (CRI) value of 86.7, a low correlated color temperature (CCT) value of 4444 K, and good heating/cooling circulation. In addition, the LED devices fabricated with this novel warm white light phosphor have excellent warm white light quality even at high temperatures, which is necessary for its practical application.

Efficient resistance against solid-state quenching of carbon dots towards white light emitting diodes by physical embedding into silica

To overcome self-quenching of carbon dots (CDs) in solid state and provide a universal strategy for efficient luminescence of CDs in solid state, this paper reports a novel method of physical embedding of solid-state CDs into a silica matrix. CD/silica composites were prepared through injecting N-(3-(trimethoxysilyl) propyl) ethylenediamine into a CD aqueous solution and have exhibited a high quantum yield of 41.72%. A mechanism of photoluminscence (PL) quenching in solid-state CDs and PL resuming in CD/silica composites was proposed. These CD/silica composites possess excellent film-forming ability, thermostability and ultraviolet (UV) stability. By taking advantage of these remarkable properties, a white light-emitting diode was constructed by combining CD/silica film with a UV chip, which has exhibited warm white light emission with Commission Internationale de L'Eclairage coordinates of (0.44, 0.42) and correlated color temperature of 2951 K. It is evident that CD/silica composites have superior potential in solid-state lighting system.

Development of sodium-zinc phosphate glasses doped with Dy3+, Eu3+ and Dy3+/Eu3+ for yellow laser medium, reddish-orange and white phosphor applications

A spectroscopy analysis of Eu2O3 (2.0mol%) or Dy2O3 (2.0mol%) singly-doped and Dy2O3 (2.0mol%)/Eu2O3 (2.0mol%) codoped sodium-zinc phosphate glasses is performed through their Raman, absorbance and photoluminescence spectra, and decay times. The phonon energy (1124cm−1) and electron–phonon coupling strength (0.028) of the glass host were determined using phonon side band spectroscopy. Laser parameter values (stimulated emission cross section, effective bandwidth, gain bandwidth and optical gain), obtained for the 4F9/2 → 6H13/2 yellow emission of the Dy3+ singly-doped glass, are higher than those reported for others 2mol% Dy2O3-doped oxide glasses, so that the magnitudes of such parameters could be suitable for a low threshold and a high gain of yellow laser emission. Additionally a value of the quantum efficiency close to the unity (0.96) was found for the dysprosium 4F9/2 level luminescence. The intense yellow emission relative to the 4F9/2 → 6H15/2 blue one leads to the Dy3+ singly-doped glass to emit neutral white light of 4647K upon 349nm excitation. The Eu3+ singly-doped glass can emit reddish-orange light of 2166K and red color purity of 97.7% upon 394nm excitation, whose (0.642, 0.351) CIE1931chromaticity coordinates are close to those (0.67, 0.33) of the red phosphor proposed by the National Television Standard Committee. The Dy3+/Eu3+codoped glass emission color can be tuned from warm white light of 3628K upon 349nm excitation to reddish-orange light of 1891 K after 364nm excitation. The warm white light emission is generated mainly by the transitions 4F9/2 → 6H13/2, 6H15/2 of Dy3+ and 5D0 → 7F2 of Eu3+, where Eu3+ is sensitized by Dy3+ through non-radiative energy transfer with an efficiency of 0.22. An electric dipole-dipole interaction into Dy3+ - Eu3+ clusters might be the dominant mechanism in the Dy3+ to Eu3+ energy transfer. Excitations at 349, 364 and 394nm can be obtained with AlGaN, GaN and InGaN based LEDs, respectively, and therefore Dy3+ and/or Eu3+ doped sodium-zinc phosphate glasses pumped by UV LEDs could be appropriate for solid state lighting technology as neutral/warm white and reddish-orange light sources.

High-efficiency/CRI/color stability warm white organic light-emitting diodes by incorporating ultrathin phosphorescence layers in a blue fluorescence layer

Abstract By incorporating ultrathin (<0.1 nm) green, yellow, and red phosphorescence layers with different sequence arrangements in a blue fluorescence layer, four unique and simplified fluorescence/phosphorescence (F/P) hybrid, white organic light-emitting diodes (WOLEDs) were obtained. All four devices realize good warm white light emission, with high color rending index (CRI) of >80, low correlated color temperature of <3600 K, and high color stability at a wide voltage range of 5 V–9 V. These hybrid WOLEDs also reveal high forward-viewing external quantum efficiencies (EQE) of 17.82%–19.34%, which are close to the theoretical value of 20%, indicating an almost complete exciton harvesting. In addition, the electroluminescence spectra of the hybrid WOLEDs can be easily improved by only changing the incorporating sequence of the ultrathin phosphorescence layers without device efficiency loss. For example, the hybrid WOLED with an incorporation sequence of ultrathin red/yellow/green phosphorescence layers exhibits an ultra-high CRI of 96 and a high EQE of 19.34%. To the best of our knowledge, this is the first WOLED with good tradeoff among device efficiency, CRI, and color stability. The introduction of ultrathin (<0.1 nm) phosphorescence layers can also greatly reduce the consumption of phosphorescent emitters as well as simplify device structures and fabrication process, thus leading to low cost. Such a finding is very meaningful for the potential commercialization of hybrid WOLEDs.

High efficiency warm white phosphorescent organic light emitting devices based on blue light emission from a bipolar mixed-host

In this study, we demonstrate a high-efficiency and low turn-on voltage warm white phosphorescent organic light emitting devices (PH-WOLEDs) based on a blue mixed-host emission layer (EML) and an orange ultrathin layer. The device has a simple structure and would simplify the fabrication process and reduce fabrication costs. The concept is based on the design a high-efficiency blue mixed-host EML, using an electron-transport material, 4,6-Bis(3,5-di(pyridin-4-yl) phenyl)-2-(3-(pyridin-3-yl) phenyl) pyrimidine (B4PYMPM) to enhance the carrier balance ability of the hole-transport material 1,3-Bis(carbazol-9-yl) benzene (MCP) which operates as the mixed-host and when the MCP: B4PYMPM ratio in the mixed-film was 4:1 got better effects. Based on the blue EML, we realized WOLEDs, characterized by a peak power efficiency of 71.3 lm/W at 3.1 V and a low turn-on voltage of 2.65 V. The mixed-host blue EML exhibited a much higher performance compared to the MCP host. Stable warm white light emission with Commission International de L'Eclairage (CIE) coordinates from (0.37, 0.45) to (0.38, 0.47) for a luminance value ranging from 1000 to 10,000 cd/m2 was obtained.

Structure, energy transfer and tunable photoluminescence of Sr 7 Zr(PO 4 ) 6 :Tb 3+ , Eu 3+ phosphors for near-UV white LEDs

Abstract A series of Sr7Zr(PO4)6:Tb3+, Eu3+ phosphors of eulytite-type structure have been synthesized via a high temperature solid-state reaction. The crystal structure and luminescence property of the samples was systematically investigated by XRD Rietveld refinement, excitation and photoluminescence, respectively. For Tb3+ and Eu3+ ions single doped phosphor, the characteristic emission from f-f transitions of Tb3+ at 544 nm and Eu3+ at 613 nm was observed, while for Tb3+ and Eu3+ co-doped phosphor Sr7Zr(PO4)6:Tb3+, xEu3+, various emission color from green to red can be achieved by trimming the relative ratio of Eu3+ to Tb3+ in Sr7Zr(PO4)6 host. Moreover, a warm white light emission of single phased with CIE coordinates at (0.359, 0.327) and correlated color temperature (CCT) at 4264 K was realized under 374 nm excitation. The energy transfer process from Tb3+ to Eu3+ in Sr7Zr(PO4)6 was verified by fluorescence spectra and decay time, where a resonant type via a dipole-dipole mechanism was demonstrated. In addition, the Sr7Zr(PO4)6:Tb3+, Eu3+ phosphors exhibited an excellent thermal quenching luminescence property owing to the structural stability. These results indicate that Sr7Zr(PO4)6:Tb3+, Eu3+ could be anticipated as color-tunable and direct white-light-emitting phosphor for near-UV-pumped w-LEDs application.

Color–tunable luminescence and energy transfer behaviors of Dy3+/Eu3+ co–doped SrLaMgTaO6 phosphors for solid state lighting applications

A series of color–tunable SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphors were synthesized via solid–state reaction. The X–ray diffraction, diffuse reflectance spectra, luminescent spectra, lifetimes and thermal quenching were applied to characterize the obtained phosphors. The X–ray diffraction results revealed that the obtained phosphors possessed pure monoclinic phase. The band gap of SrLaMgTaO6 was calculated from Vienna ab initio simulation package and diffuse reflectance spectra. The energy transfers from host to Dy3+ and Eu3+ ions were confirmed by the luminescence spectra. Furthermore, the SrLa0.92MgTaO6: 0.07Dy3+, 0.01Eu3+ phosphor had excellent thermal stability, while the integrated PL intensity still kept about 83.34% at 150°C of that measured at room temperature (30°C). In addition, the warm and cold white light emission could be generated with different Eu3+ contents. All the results of SrLa0.93-yMgTaO6:0.07Dy3+, yEu3+ phosphors show great potential as a single–phase white–emitting phosphor for solid state lighting applications.

Design of Phosphor White Light Systems for High-Power Applications

We developed a strategy that transforms phosphor down-converting white light sources from low-power systems into efficient high-power ones. To incorporate multiple phosphors, we generalized and extended a phosphor layer model, which we term CCAMP (color correction analysis for multiple phosphors). CCAMP describes both the scattering and saturation of phosphor materials and allows modeling of different layered structures. We employed a phosphor mixture comprising YAG:Ce and K2TiF6:Mn4+ to illustrate the effectiveness of the model. YAG:Ce’s high density and small particle size produce a large amount of scattering, while the long (4.8 ms) photoluminescent lifetime of K2TiF6:Mn4+ results in saturation at high pump power. By incorporating experimental photophysical results from the phosphors, we modeled our system and chose the design suitable for high-power applications. We report the first solid-state phosphor system that creates warm white light emission at powers up to 5 kW/cm2. Furthermore, at this high p...

Solvent‐Polarity‐Engineered Controllable Synthesis of Highly Fluorescent Cesium Lead Halide Perovskite Quantum Dots and Their Use in White Light‐Emitting Diodes

Cesium lead halide quantum dots (QDs) have tunable photoluminescence that is capable of covering the entire visible spectrum and have high quantum yields, which make them a new fluorescent materials for various applications. Here, the synthesis of CsPbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) QDs by direct ion reactions in ether solvents is reported, and for the first time the synergetic effects of solvent polarity and reaction temperature on the nucleation and growth of QDs are demonstrated. The use of solvent with a low polarity enables controlled growth of QDs, which facilitates the synthesis of high‐quality CsPbX3 QDs with broadly tunable luminescence, narrow emission width, and high quantum yield. A QD white LED (WLED) is demonstrated by coating the highly fluorescent green‐emissive CsPbBr3 QDs together with red phosphors on a blue InGaN chip, which presents excellent warm white light emission with a high rendering index of 93.2 and color temperature of 5447 K, suggesting the potential applications of highly fluorescent cesium lead halide perovskite QDs as an alternative color converter in the fabrication of WLEDs.

Fabrication of hybrid white OLEDs with an inorganic color conversion layer

ABSTRACTWe report hybrid white organic light-emitting diodes (WOLEDs) made by combining FIrpic-based blue-emitting OLEDs and a printable red-emitting inorganic phosphor. The color conversion can be easily controlled by adjusting the thickness of the red phosphor. FIrpic-based phosphorescent OLEDs, with a 15-μm-thick phosphor layer, show warm white light emission leading to a current efficiency of 11.4 cd/A, a power efficiency of 4.9 lm/W, and a CIE coordinate of (0.33, 0.41) at a luminance of 1000 cd/m2. Moreover, the output spectra and CIE coordinates of the WOLED show no significant change over a wide range of current densities.

Warm white light generation from single phase Sr3Y(PO4)3:Dy3+, Eu3+ phosphors with near ultraviolet excitation

Novel Sr3Y(PO4)3:Dy3+, Eu3+ (SYP:Dy3+, Eu3+) phosphors were synthesized by a standard solid-state reaction under an ambient air atmosphere and their structural and optical properties were investigated. XRD and diffuse reflectance spectra (DRS) were used to explore structural properties. The former showed that single phase phosphors were obtained and that the rare earth ions entered into the cubic host by substituting the smaller Y3+ ions and thereby enlarging the unit cell. The DRS indicated that the host has a direct bandgap of 4.5eV. Under 393nm excitation, a strong and stable warm white light emission with high color purity was achieved in SY0.92P:0.06Dy3+, 0.04Eu3+. The energy transfer from Dy3+ to Eu3+ ions was investigated and the related mechanism was discussed based on the optical spectra and emission decay curves. The thermal quenching of emission is similar to that of YAG:0.06Ce3+. The results show the single phase phosphor is potential in warm white LED.

White light emission from novel host-sensitized single-phase Y2WO6: Ln3+ (Ln3+=Eu3+, Dy3+) phosphors

A series of Eu3+- or Dy3+-doped and Eu3+/Dy3+ co-doped Y2WO6 in pure phase was synthesized via high-temperature solid-state reaction. X-ray diffraction, diffuse reflection spectra, photoluminescence excitation and emission spectra, the CIE chromaticity coordinates and temperature-dependent emission spectra were exploited to investigate the phosphors. Upon UV excitation at 310nm, efficient energy transfer from the host Y2WO6 to dopant ions in Eu3+ or Dy3+ single-doped samples was demonstrated and those phosphors were suitable for the UV LED excitation. The intense red emission was observed in Y2WO6: Eu3+, and blue and yellow ones were observed in Y2WO6: Dy3+. Concentration quenching in Y2WO6: Dy3+ phosphors could be attributed to the electric dipole-dipole interaction. In Eu3+/Dy3+ co-doped Y2WO6 phosphors energy transfer process only took place from the host to Eu3+/Dy3+ ions and warm white-light emission can be obtained by adjusting the dopant concentrations. The temperature-dependent luminescence indicated Eu3+/Dy3+ co-doped Y2WO6 was thermally stable. Our overall results suggested that Y2WO6: Ln3+ (Ln3+=Eu3+, Dy3+) as warm white-light emitting host-sensitized phosphor might be potentially applied in WLEDs.

Cool to warm white light emission from hybrid inorganic/organic light-emitting diodes

The synthesis and characterisation of two novel organic down-converting molecules is disclosed, together with their performance as functional colour-converters in combination with inorganic blue light-emitting diodes (LEDs).

Long-wave UVA radiation excited warm white-light emitting NaGdTiO4: Tm3+/Dy3+/Eu3+ ions tri-doped phosphors: Synthesis, energy transfer and color tunable properties

NaGdTiO4 (NGT) phosphors doped with different activator ions (Tm3+, Dy3+, and Eu3+) were synthesized by a conventional solid-state reaction method in an ambient atmosphere. These phosphors were characterized by scanning electron microscope images, transmission electron microscope images, X-ray diffraction patterns, Fourier transform infrared spectra, and photoluminescence spectra. All the samples were crystallized in an orthorhombic phase with a space group of Pbcm (57). In Tm3+/Dy3+ ions co-doped samples, white-light emission was observed under near-ultraviolet (NUV) excitation. In addition, the energy transfer between Tm3+ and Dy3+ ions was proved to be a resonant type via an electric dipole–dipole mechanism and the critical distance of energy transfer was calculated to be 19.91 Å. Furthermore, Tm3+/Dy3+/Eu3+ ions tri-doped NGT phosphors demonstrated warm white-light emission by appropriately tuning the activator content, based on the principle of energy transfer. These NUV wavelength excitable phosphors exhibit great potential as a single-phase full-color emitting phosphor for white light-emitting diode applications.

Pressure-Stimulated Synthesis and Luminescence Properties of Microcrystalline (Lu,Y)₃Al₅O₁₂:Ce³⁺ Garnet Phosphors.

The Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 phosphors were synthesized by solid state reaction at temperature 1623 K and pressure 1.5 × 10(7) Pa in (95% N2 + 5% H2) atmosphere. Under the conditions, the compounds crystallize in the form of isolated euhedral partly faceted microcrystals ∼19 μm in size. The crystal structures of the Lu2.98Ce0.01Y0.01Al5O12 and Y2.99Ce0.01Al5O12 garnets have been obtained by Rietveld analysis. The photoluminescence (PL) and X-ray excited luminescence (XL) spectra obtained at room temperature indicate broad asymmetric bands with maxima near 519 and 540 nm for Y2.99Ce0.01Al5O12 and Lu2.98Ce0.01Y0.01Al5O12, respectively. The light source was fabricated using the powder Lu2.98Ce0.01Y0.01Al5O12 phosphor and commercial blue-emitting n-UV LED chips (λ(ex) = 450 nm). It is found that the CIE chromaticity coordinates are (x = 0.388, y = 0.563) with the warm white light emission correlated color temperature (CCT) of 6400 K and good luminous efficiency of 110 lm/W.

On the efficient warm white-light emission from nano-sized Y2O3

We consider the reported emission of white light (WL) in the spectral range from 400 to beyond 900nm induced by monochromatic infrared light (803.5 and 975nm) continuous wave excitation of nominally un-doped yttrium oxide (Y2O3) nano-powders. Based on the experimental evidence, such an emission feature is a nano-scale phenomenon, resembles very closely the emission from an incandescent lamp (mimicking the sunlight, i.e., the most comfortable light to human eyes) and exhibits very high efficiency (864lum/W) and nearly theoretical (i.e., 99) color rendering index. At the fundamental level, the origin of this phenomenon is still unexplained. In this paper we address the fundamental questions raised by the reported occurrence of WL emission from Y2O3 nanopowders and attempt an interpretation at a more fundamental level. In particular we focus on the multiphoton-absorption and nonexponential decay patterns of the reported WL emission as starting points to formulate models and interpretations of the experimental occurrences still lacking in the literature. Our discussion invokes the electronic dispersion of Y2O3 and nanoscale effects, which is supported by the experimental evidence according to which the observed warm WL emission is a nanoscale phenomenon with properties that only can be explained by nanoscale physics.

White light generation in Dy3+-and Ce3+/Dy3+-doped zinc–sodium–aluminosilicate glasses

A spectroscopic investigation of 1% Dy2O3-singly doped and 0.5% Ce2O3-1.0% Dy2O3-codoped zinc–sodium–aluminosilicate glasses was performed by analyzing their absorption and photoluminescence spectra, and decay times. Warm white yellow light emission, with (0.419, 0.440) CIE1931 chromaticity coordinates and 3579K color temperature, is obtained in the Dy3+-singly doped glass excited at 399nm, which fits to the requirements of GaN LEDs. A quantum efficiency of 74% and a very high optical gain (38.7×10−25cm2s) were estimated for the dysprosium 4F9/2 level luminescence, which might also make the Dy3+-doped glass a promising gain medium for solid state yellow laser pumped by GaN LEDs. In the Ce3+/Dy3+-codoped glass a radiative energy transfer from Ce3+ to Dy3+ is observed upon UV excitation (310–365nm), with a Ce3+ to Dy3+ interaction distance that could be greater than 6–12Å. The emission color from the codoped glass can be tuned with the excitation wavelength from blue light (0.247, 0.245), upon 310nm excitation, to cold white light (0.284, 0.300), with a 9052K color temperature, upon 365nm excitation.

The role of neutral and ionized oxygen defects in the emission of tin oxide nanocrystals for near white light application

Tin oxide (SnO2) nanocrystals (NCs) based phosphor was synthesized by a green chemistry microwave-assisted hydrothermal method at different reactor pressures. The x-ray diffraction analysis showed that a single rutile SnO2 phase with a tetragonal lattice structure was formed. The photoluminescence emission was measured for He–Cd laser excitation at 325 nm and it showed a broad band emission from 400 to 800 nm for all the synthesized reactor pressures. The broad emission spectra were due to the creation of various oxygen and tin defects as confirmed by x-ray photoelectron spectroscopy data. The origin of the emission in the SnO2 NCs is discussed with the help of an energy band diagram. Analysis suggests that the visible emission of SnO2 NCs is due to a transition of an electron from a level close to the conduction band edge to a deeply trapped hole in the SnO2 NCs. The NCs were found to be suitable for warm near white light emission device applications.